This invention relates to mutant tobacco plants having agronomically useful genetic resistance to certain broad-spectrum herbicides.
Chaleff, Science 219:676 (1983) discusses methods of isolating plant mutants from tissue culture under selective growth conditions. These methods have been applied to several crop species, but the majority of novel phenotypes reported to date are of limited practical utility. These results can be attributed to the genetic and developmental complexity of agriculturally important characteristics. Many traits are exclusively whole plant functions which are not expressed at the cellular level, and thus cannot be detected by an in-vitro selection procedure.
Herbicide resistance resulting from alterations of basic metabolic functions is a trait which can be expressed by isolated plant cells in culture. since herbicides that inhibit such functions can be expected to inhibit growth of cultured cells as well as developed plants. Chaleff, et al., Proc. Nat. Acad. Sci. USA 75:5104-5107 (1978) describe isolation of tobacco mutants exhibiting increased tolerance to picloram (4-amino-3,5,6-trichloropicolinic acid), a systemic herbicide. Picloram tolerance was expressed by regenerated plants and transmitted to progeny as a dominant or semidominant nuclear allele. Seeds derived from picloram-tolerant mutants germinated in the presence of 100 .mu.M picloram. However, development beyond the seedling stage was not observed. After one month's exposure to 50 .mu.M picloram, plantlets homozygous for tolerance mutations developed stem callus tissue and degenerated. However, the effect of picloram on soil-grown picloram-tolerant strains was not determined.
Miller, et al., In Vitro 16:1085-1091 (1980) describe isolation of paraquat-resistant cell lines from cell cultures of Nicotiana tabacum. Plants were regenerated from resistant callus cultures, but no breeding experiments were conducted to study inheritance of the paraquat resistance trait. In any event, the resistance reported was a short-lived phenomenon. In these experiments, plants were considered "resistant" if leaves required 4-7 days to bleach to white at paraquat concentrations of 10.sup.-5 M, versus an average of 3.5 days for controls. The authors concluded that several forms of paraquat resistance could be detected in vitro, not all of which were expressed by intact plants.
Singer, et al., Plant Physiol. 69:573 (1982) describe isolation and regeneration of tobacco plants exhibiting tolerance to amitrole (3-amino-1,2,4-triazole) and glyphosate (N-phosphonomethyl glycine). For the majority of isolates, tolerance was diminished or eliminated after lengthy passages away from herbicide. In the few cases in which inheritance of tolerance was observed, Singer, et al., described the patterns of such inheritance as "complex."
Thomas, et al., Theor. Appl. Genet. 63:169-176 (1982) describe isolation of paraquat-tolerant mutants from tomato cell cultures. Diploid plants were regenerated from several of the paraquat-tolerant clones. Some of the plants appeared normal, but others had altered morphology and reduced vigor and fertility.
The potential of in vitro selection techniques for development of herbicide-resistant crop varieties has been highlighted by Knopf, J. Theor. Biol. 94:985 (1982). Knopf proposes a "plant for the herbicide" concept, involving development of crop varieties specifically resistant to a selected herbicidal compound with "ideal" characteristics. Such a compound should exhibit a wide spectrum of effectiveness at low rates of application, in combination with low animal toxicity.
Many N-(heterocyclicaminocarbonyl)arylsulfonamides exhibit very potent, broad-spectrum herbicidal activities and low mammalian toxicities. Exemplary of this class of herbicides are chlorsulfuron and sulfometuron-methyl, the active ingredients of the Du pont herbicides Glean.RTM. and Oust.RTM., respectively.
Accordingly, a principal object of the present invention is to provide mutant strains of Nicotiana tabacum with agronomically useful resistance to herbicidal sulfonamide compounds. A further object is to provide methods of obtaining the mutant strains. Such mutants should be stable with respect to the resistance phenotype throughout the plant life cycle, and be capable of genetically transmitting the resistance trait to progeny in predictable fashion, enabling integration of the resistance trait into commercial varieties by means of breeding programs.